Cathode support and heat shielding structure for electron gun



G. B. KUEHNE Filed May 2, 1962 ATTORNEY Jan. 4, 1966 GATHODE SUPPORT AND HEAT surname STRUCTURE FOR ELECTRON GUN United States Patent 3,227,906 CATHODE SUPPORT AND HEAT SHIELDING STRUCTURE FOR ELECTRON GUN Gerhard B. Kuehne, Santa Clara, Calif, assignor to Eitel- McCullough, Inc., San Carlos, Califi, a corporation of California Filed May 2, 1962, Ser. No. 191,894 11 Claims. (Cl. 313-45) This invention relates to electron guns for beam tubes, and more particularly to a cathode support and heat shielding structure for electron beam guns.

One of the diflicult problems facing the designer of a high convergence electron beam gun is the necessity of providing a cathode support structure which supports the cathode free from undesirable vibration which can cause appreciable beam interception by associated structure. In an electron gun such as that illustrated, which has a cathode-to-beam convergence ratio of about fiftyto-one and a cathode current density of about three amperes per square centimeter at a beam voltage of 21 kv., any appreciable beam interception due to vibration would be catastrophic. It is therefore one of the objects of the invention to provide a cathode support structure Which retains the cathode rigidly supported again such vibration.

Another diflicult problem in the design of an electron gun for an electron beam tube is the necessity of supporting the cathode in a manner which will minimize conduction of heat away from the cathode while simultaneously providing the requisite rigidity. It is therefore another object of this invention to provide a cathode support which minimizes the mass of the cathode support structure and therefore minimizes the conduction of heat away from the cathode without sacrificing rigidity in the support structure.

To minimize the amount of power required to maintain the cathode at a given temperature, it is important that heat be prevented from moving away from the heater in a direction away from the cathode. Since some heat will inevitably flow in a direction away from the cathode, it is another object of the invention to provide means for conducting and/or reflecting such heat back toward the cathode.

In the operation of an electron gun, it is important that spacings between electrodes be maintained at optimum value. The spacing between electrodes when they are at operating temperature is usually different from the spacing when the electrodes are at room temperature, which is usually the temperature at which they are constructed and assembled. It is therefore important that the electron gun electrodes be supported in a manner which enables substantially accurate precalculation of the extent of thermal expansion and contraction of the various electrodes and supporting members and the resultant spacing between the electrodes at operating temperature. It is accordingly a still further object of the invention to provide an electron gun supporting structure in which the cooperative relationship of the elements is such as to enable substantially accurate prediction of the direction and extent of movement of the various elements of the combination as a result of thermal expansion and contraction.

Another difliculty encountered in the fabrication of an electron beam gun is the chemical incompatibility of the various types of metals used in the fabrication of an electron gun. For instance, tantalum is less compatible with tungsten than is molybdenum and it is therefore more difficult to form a reliable autogenous or other union between tantalum and tungsten than between molybdenum and tungsten. On the other hand, tungsten possesses certain qualities which makes it preferable for use in cath: odes, whereas cathode support structures are more conveniently fabricated from tantalum because of inherent characteristics of this metal. It is therefore a still further object of the present invention to provide a tungsten matrix cathode supported by a plurality of rigid tantalum struts, the struts being secured to the tungsten cathode in a manner to ensure reliability of the union.

Another problem encountered in the operation of an electron beam tube is emission of electrons from electrodes other than the cathode. This is particularly true with respect to the focus electrode in a relatively high convergence electron beam gun such as that shown because of the close proximity of the focus electrode to the cathode and its resultant susceptibility to being contaminated and heated to high emitting temperatures. It is therefore a still further object of the present invention to provide shield means between a cathode and focus electrode so as to diminish contamination and heating of the focus electrode.

The invention possesses other objects and features of value, some of which, with the foregoin will be apparent from the following description and the drawing. It is to be understood, however, that the invention is not limited to the embodiment illustrated and described, but may be embodied in a variety of forms within the scope of the appended claims.

Briefly described, the electron gun support and shield structure of the invention comprises a hollow cylindrical support shell having supported adjacent one end thereof a matrix-type tungsten cathode having a concave emitting surface facing away from the support shell, the cathode being rigidly supported by a plurality of circumferentially spaced, axially extending metallic struts contoured to provide the requisite strength and rigidity while possessing a low mass. Each strut is fixed at one end to the adjacent end of the support shell, and at its other end is secured to a peripheral portion of the cathode. Rigidly interposed between the cathode and the support shell are a plurality of concave heat reflecting shells arranged in axially aligned and spaced succession adjacent the back side of the cathode so as to reflect back toward the cathode heat radiated from the back side of the oath ode. The heat reflecting shells are supported in position by longitudinally extending support struts of low mass which are in turn secured to the support shell so that heat from these reflecting shells is also conducted to the support shell. Since heat from the cathode also radiates outwardly from the cathode, an auxiliary heat reflecting shell having a cylindrical portion and a conical portion is provided surrounding a peripheral edge of the cathode in radially spaced relation thereabout. The cathode is preferably heated by a bifilar-type heating filament coated with an insulating layer or, alternatively, embedded in a thermally conductive insulating wafer contiguously disposed on the side of the cathode opposite the emitting surface, as shown, for eflicient heat conduction therebetween, and retained thereagainst by a metallic retaining and heat conductor shell having a peripheral flange secured to a peripheral portion of the cathode. A beamforming focus electrode is provided supported on the cathode support shell, the focus electrode including a cylindrical support portion and a re-entrant truncated conical electrode portion having an edge in close proximity to the outer edge of the emitting surface of the cathode. The cooperative relationship of cathode sup port struts, focus electrode, and auxiliary heat shield is such that thermal expansion of these members results in substantially equal amounts of movement of the various parts and in the same direction, so that their relative w.) spacings remain substantially constant through a wide range of temperature fluctuations.

Referring to the drawings:

FIGURE 1 is a vertical cross-sectional view illustrating the cathode support and shielding structure greatly enlarged over actual size and showing the cooperative relationship of the parts.

FIGURE 2 is a fragmentary cross-sectional vieW illustrating the preferred manner of locking the support struts to the peripheral edge portion of the cathode.

In more specific detail, the electron gun support and heat shielding structure of the invention comprises a hollow cylindrical support shell 2, preferably fabricated from sheet molybdenum having a thickness ranging between .010" and .015, and provided at one end with a radially outwardly extending mounting flange 3, and at the other end with longitudinally extending slots 4. The slots are preferably five in number and are circumferentially equally spaced about the shell. In a gun structure fabricated according to this invention, the slots were conveniently dimensioned so as to be .062" wide and .093" deep. The support shell was only .650 long. From this it will be apparent that another attribute of the present invention is the applicability of the design to extremely small structural units such as are required for electron guns in beam tubes operating at X-band.

Disposed adjacent the notched end of the support shell is a matrix-type impregnated tungsten cathode 6, having a concave emitting surface7, and a convex surface 8 on the back side of the cathode opposite the emitting surface. The cathode button is preferably provided with a central aperture 9 useful during fabrication for alignment purposes, and useful during operation to prevent destructive ion bombardment of the cathode. At its outer periphery the cathode is provided with a cylindrically extending integral flange 12. Formed in the circular peripheral surface 13 of the cathode is a groove 14.

The cylindrically extending flange on the cathode provides a convenient flat annular surface 16 on which the radially extending flanges 17 of a retaining and thermally conductive shell 18 may be welded or otherwise secured in an eflicient heat exchanging union. The retaining shell follows the curvature of the convex surface of the cathode and cooperates therewith to provide a heater chamber within which is confined a bifilar spiral heater fllament 19 suitably coated with a thermally conductive, electrically insulating coating or embedded as shown in a thermally conductive dielectric material 21 in the form of a Wafer to rigidly support the heater against vibration and provide conduction heating of the cathode. It has been found that a two-mil coating on a filament having a diameter of about eight to ten mils is satisfactory. The shell 18 is preferably formed from molybdenum and is concave so as to conduct heat from the heater filament back toward the peripheral edge of the cathode. Input and output leads 22 and 23 extend coaxially through the cathode support shell along the axis thereof and are electrically insulated from each other and from surrounding structure by dielectric member 24 appropriately bored to receive the leads.

To rigidly support the cathode 6 in a manner which will minimize the conduction of heat away from the cathode while simultaneously rigidly supporting the cathode against vibration, a plurality of circumferentially spaced, axially extending, elongated meallic struts 26 are provided. The struts are preferably fabricated from strip molybdenum or tantalum having a thickness of only about .002" and are formed with a longitudinally extending rib 27 for added rigidity. Because of their low mass, the struts effectively minimize conduction of heat away from the cathode. At one end 28 the struts are united to the cathode support shell at a point thereon spaced from the end thereof, while at the opposite end 29 each strut is provided with an axially extending flange portion 31 spot-welded to the peripheral surface 13 of the cathode.

For added reliability in its securement to the cathode,

the strut is provided with a radially inwardly extending flange 32 terminating in another axially extending flange 33. The flanges 32 and 33 are proportioned to lie snugly within groove 14. A lock ring 34 is provided superimposed over the flanges 32 and 33 of the struts, and is suitably welded to the cathode itself. If desired, the lock ring may additionally be welded to each flange 33. The lock ring is preferably formed from a length of molybdenum Wire of a gauge to closely bind the flanges 32 and 33 within the groove. Since it is diflicult during fabrication to maintain close tolerances, any discrepancies in proportions which may result in a loose fit between flanges and grooves may be compensated by selection of an appropriately sized wire.

To assemble the cathode on the flanged ends of the struts, the flanges are preferably appropriately engaged in the groove at circumferentially equally spaced intervals and tack-welded as at 31. The molybdenum rock ring, which initially comprises an indeterminate length of wire, is then wound about the cathode and superimposed over the flanges 33 and appropriately spot-welded in place, the excess wire being cut olf. The other ends of the struts are welded to the support shell 2, the proportions of the parts being such that the struts converge inwardly toward the cathode, and in so doing, pass through the slots 4 formed in the edge of the support shell 2. The width of the support struts is proportioned so that each strut fits snugly within the associated slot, so as to provide lateral or edgewise support for the struts intermediate their ends, thus ensuring that vibration is minimized or does not occur. The cathode is of course appropriately jigged during attachment to the shell 2 so that the cathode lies perpendicularly across the end of the support shell but axially spaced therefrom.

To intercept and reflect heat from the heater back toward the cathode, a plurality of concave heat reflecting shells 36, 37, and 38 are provided interposed between the heater and the cathode support shell. Each heat reflecting shell is provided with a concave reflecting surface 39 and an integral cylindrical peripheral support portion 41, the cylindrical support portion 41 of shell 36 which lies next adjacent the cathode being provided with a plurality of circumferentially equally spaced slots 42, corresponding in spacing and number to the struts supporting the cathode.

The cathode is therefore surrounded about its outer periphery 13 by the cylindrical porton 41 of the shell 36, except at the occurrence of each of the slots 42.

As shown in FIGURE 1, each shell is provided with a central aperture 43 through which the dielectric rod 24 passes. The concave reflecting shells are preferably supported in axially spaced relation with respect to each other and in radially spaced relation with respect to the support shell 2 by a set of strip-like support struts 44, each formed from .010 tantalum, and provided intermediate its ends with transverse beads or ridges 46, useful in establishing the axial position of shells 37 and 38. As shown in FIG- URE 1, the cylindrical portions 41 of these shells abut the beads 46 and are thus appropriately positioned with respect to each other, and with respect to shell 36. It has been found that five struts are a preferred number to rigidly retain the shells in position. The concave heat reflecting shells are preferably assembled with the struts as a subassembly, each of the shells being welded to the cylindrical surface 41.

The assembly may then be inserted into position and spot-Welded to the support shell. The subassembly is appropriately positioned when the edge of the cylindrical portion 41 on concave shell 36 lies flush with the surface of the cathode button on the side thereof next adjacent emitting surface 7. From this construction, it will be apparent thatheat flowing axially away from the heater is first intercepted and conducted back to the cathode by the retaining shell 18. Whatever heat passes or is radiated from the shell 18 is successively intercepted and reflected from the concave shells 36, 37 and 38. This arrangement of heat shields has been found to be effective in minimizing heat loss due to conduction and radiation from the heater. I have found it convenient to fabricate all of the concave reflecting shells from .005" sheet molybdenum.

It is also desirable to prevent the flow of heat radially outwardly from the cathode. In the present construction this is to some extent effected by the cylindrical portion 41 on shell 36, which surrounds the peripheral surface 13 of the cathode in radially spaced relation. Because of the slots 42, however, an appreciable amount of heat is radiated past the portion 41. To intercept such heat, an auxiliary heat shield is provided having a cylindrical shell portion 47 and a truncated conical shell portion 48 having a central opening 49 therein. One end of the cylindrical shell portion 47 is welded adjacent the notched end of the support shell 2, surrounding the cathode and cylindrical portion 41 of concave reflecting shell 36 in radially spaced relation thereabout. The truncated conical shell portion 48 tapers inwardly into close proximity with the emitting surface of the cathode.

The cylindrical shell portion 47 therefore eflectively intercepts and reflects back toward the cathode any heat that radiates outwardly from the heater and cathode. The conical shell portion 48 cooperates by intercepting and radiating back towards the cathode such heat as is radiated from the next adjacent surfaces of the cathode. It will thus be seen that the concave and cylindrical heat shields cooperate to intercept and reflect heat back toward the cathode, and thus increase the efliciency of the heater-cathode package. The auxiliary heat shield is conveniently fabricated from .010" tantalum. It should be noted that since the adjacent or corresponding ends of the cathode support struts and the auxiliary heat reflecting shell 47 are fixed to the same end of the cathode support shell, thermal expansion of these members will occur in the same direction and to substantially the same extent, thus ensuring that the inner peripheral edge 49 of the auxiliary conical heat shell portion will remain in approximately the same spaced relationship with associated elements through a wide range of temperature fluctuations.

Cooperating with the cathode to provide the proper convergence of the electrons projected therefrom, is a focus electrode assembly 51 including a cylindrical support shell 52 having a radially outwardly extending flange 53 at one end welded on a complementary flange 54 on support shell 2 intermediate its ends. Adjacent its other end the shell 52 is provided with a radially inwardly extending conical portion 56 terminating in a cylindrical flange portion 57 closely surrounding and fixed to the relatively heavy tubular focus electrode portion 58. The portion 58 has a re-entrant conical portion 59 extending toward the emitting surface of the cathode in appropriately close proximity to the outer edge thereof and in close proximity to the apertured apex end 49 of the conical shield portion 48.

The relationship and proportion of the parts is such that the conical portion of the focus electrode extends through the apertured apex end of the shield 48, which closely surrounds the outer peripheral portion of the conical focus electrode portion 59, thus functioning to shield the focus electrode from heat radiated from the cathode and other contaminants that might be deposited on the focus electrode. As with the cathode struts 26 and the auxiliary cylindrical heat shield 47, the cylindrical focus electrode support shell 52 is fastened intermediate the ends of the support shell 2 and will expand and contract in the same direction as the support struts 26 and 44, thus ensuring that the electrode spacing between the focus electrode and cathode will be maintained.

From the foregoing it will be apparent that the electron gun cathode support and shield assembly lends itself to fabrication by mass production techniques in separate subassemblies easily assembled into a composite structure. This is important in electron gun structures, such as that illustrated, in which the cathode is a scant one-half inch in diameter, and the remaining parts are correspondingly small. It will of course be readily recognized that as the size of the unit diminishes, the complexity of its fabrication and assembly increases, thus increasing the over-all cost of the product. For this additional reason, it is important that structural relationships be utilized which are capable of being scaled down so as to increase the economy of fabrication.

I claim:

1. In an electron gun including a cathode having a peripheral edge portion, a cathode support structure comprising a plurality of elongated circumferentially spaced support struts each having an end portion engaging the peripheral edge portion of the cathode, and a lock ring superimposed over the end portions of the struts to lock the struts to the cathode.

2. The combination according to claim 1, in which the peripheral edge portion of the cathode is provided With a groove, the end portion of each said strut is provided with a flange seated in the groove, and the lock ring lies within the groove to lock the strut flanges therein.

3. In an electron gun including a cathode having a heater operatively associated therewith, the combination of a cathode support structure and a radiant heat shielding assembly comprising a hollow support shell having one end adjacent the cathode, a first set of circumferentially spaced axially extending support struts supportingly interposed between the cathode and the associated end portion of the support shell, a radiant heat reflecting shell interposed between the cathode and support shell and having a concave reflecting surface arranged to intercept heat waves radiated axially toward the support shell and reflect them back toward the cathode, and a second set of circumferentially spaced axially extending support struts supportingly interposed between the radiant heat reflecting shell and the support shell to retain the reflecting shell spaced from the support shell.

4. The combination according to claim 3, in which, each of said first set of support struts having a ribbed portion, the radiant heat shielding assembly includes a succession of spaced heat reflecting shells each having a concave reflecting surface arranged to reflect heat toward the cathode, the heat reflecting shells being supported in spaced relation to each other and to the support shell by the second set of support struts.

5. The combination according to claim 3, in which an auxiliary heat reflecting shell is disposed about the cathode to intercept heat waves radiated away therefrom in a radial direction, said auxiliary heat reflecting shell including a cylindrical portion fixed to the support shell and having at its other end a truncated conical portion the apex end of which lies closely adjacent the emitting surface of the cathode.

6. The combination according to claim 5, in which a focus electrode is provided comprising a cylindrical shell fixed at one end on the hollow cathode support shell and having at its other end a re-entrant portion extending through the apex end of truncated conical portion of said auxiliary heat reflecting shield into close proximity to the emitting surface of the cathode.

7. In an electron gun including a cathode having a heater operatively assoicated therewith, the combination comprising a cathode support structure; a radiant heat shielding structure; and a focus electrode structure; said cathode support structure including a hollow support shell having one end adjacent the cathode, and means interposed between the cathode and support shell to rigidly support the cathode thereon; said radiant heat shielding structure including a shell interposed between the cathode and support shell and having a concave reflecting surface arranged to intercept heat waves radiated axially toward the support shell and reflect them back toward the cathode, said heat reflecting shell interposed between the cathode and support shell being provided with apertures through which said cathode support means pass, and means interposed between the heat' reflecting shell and the support shell to retain the reflecting shell spaced from the support shell; and said focus electrode structure includes a hollow shell having a cylindrical support portion and an inwardly tapering truncated conical electrode portion, said cylindrical focus electrode support portion being secured to said support shell intermediate the ends of the latter, and said truncated conical electrode portion at its apex end lying closely adjacent the cathode.

8. The combination according to claim 7, in which an auxiliary heat reflecting shell is mounted on said hollow shell and is disposed about the cathode to intercept heat waves radiated away therefrom in a radial direction, said auxiliary heat reflecting shell including a truncated conical portion tapering in the same direction as the truncated conical electrode portion of the focus electrode, the apex ends of both said truncated conical portions lying in close proximity to the emitting surface of the cathode.

9. In an electron gun including a cathode, a cathode support structure comprising a hollow support shell having one end adjacent the cathode, and a plurality of circumferentially spaced axially extending support struts supportingly interposed between the cathode and the associated end portion of the support shell, one end of each said support strut being secured to the support shell at a point spaced from the associated end thereof, said struts being inclined toward the cathode and secured to a peripheral edge portion thereof,

10. In an electron gun including a cathode, a cathode support structure comprising a hollow support shell having one end adjacent the cathode and a plurality of ribbed support struts supporting said cathode on said hollow support shell, said ribbed support struts being circumferentially spaced at said one end of said hollow support shell, said ribbed support struts being inclined toward said cathode and secured to a peripheral edge portion thereof.

11. The combination according to claim 10 in which said plurality of ribbed support struts consist of five struts each having a thickness of about .002 inch.

References Cited by the Examiner UNITED STATES PATENTS Re. 22,378 9/ 1943 Bowie.

2,093,876 9/1937 Von Ardenne. 2,268,165 12/1941 Parker et al. 2,808,527 10/ 1957 McKenzie 313-82 2,912,616 11/1959 Marchese et al. 31382 X 2,916,659 12/1959 Sege. 2,967,260 1/ 1961 Eitel. 2,992,347 7/1961 Dehn 313--82 3,056,900 10/1962 Tyson. 3,065,374 11/1962 Rockwell 315-538 X 3,132,275 5/1964 Merdinian 313-82 FOREIGN PATENTS 898,193 11/1953 Germany.

References Cited by the Applicant UNITED STATES PATENTS 2,815,467 12/1957 Gardner. 2,971,115 2/1961 Nelson.

GEORGE N. WESTBY, Primary Examiner.

ARTHUR GAUSS, ROBERT SEGAL, Examiners. 

1. IN AN ELECTRON GUN INCLUDING A CATHODFE HAVING A PERIPHERAL EDGE PORTION, A CATHODE SUPPORT STRUCTURE COMPRISING A PLURALITY OF ELONGATED CIRCUMFERENTIALLY SPACED SUPPORT STRUTS EACH HAVING AN END PORTION ENGAGING THE PERIPHERAL EDGE PORTION OF THE CATHODE, AND A LOCK RING SUPERIMPOSED OVER THE END PORTIONS OF THE STRUTS TO LOCK THE STRUTS TO THE CATHODE. 